276 Evolution and Adaptation 



has directed the formation of the new species would not, of 

 course, be shown, nor would it make any difference in the 

 present connection. 



Before we attempt to reach a conclusion on this point let 

 us analyze the facts somewhat more closely. 



In the first place, a number of these cases of discontinuous 

 variation are of the nature of abnormalities. The appearance 

 of extra fingers or toes in man and other mammals is an ex- 

 ample of this sort. This abnormality is, if inherited at all, 

 inherited completely ; that is, if present the extra digit is per- 

 fect, and never appears in an intermediate condition, even 

 when one of the parents was without it. The most obvious 

 interpretation of this fact is that when the material out of 

 which the fingers are to develop is divided up, or separated 

 into its component parts, one more part than usual is laid 

 down. Similarly, when a flower belonging to the triradiate 

 type gives rise to a quadriradiate form, — ■ as sometimes 

 occurs, — the new variation seems to depend simply on the 

 material being subdivided once more than usual ; perhaps 

 because a little more of it is present, or because it has a 

 somewhat different shape. My reasons for making a sur- 

 mise of this sort are based on certain experimental facts in 

 connection with the regeneration of animals. It has been 

 shown in several cases that it is possible to produce more 

 than the normal number of parts by simply dividing the ma- 

 terial so that each part becomes more or less a new whole, 

 and the total number of parts into which the material becomes 

 subdivided is increased. It seems not improbable that phe- 

 nomena of this sort have occurred in the course of evolution, 

 although it is, of course, possible that those characters that 

 define species do not belong to this class of variation. To 

 take an example. There are nine neck-vertebroe in some 

 birds, but in the swan the number is twenty-five. We cannot 

 suppose that the ancestor of the swan gradually added enough 

 materially to make up one new vertebra and then another, 



